Team invitation link returns "Forbidden" instead of acceptance page

### Is this a hackmd.io issue?
- [x] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### What's the problem

#### Current behaviour
I received a team invitation email from HackMD. When I click the "Accept invitation" link in the email, the browser opens a page on https://hackmd.io that simply says "Forbidden". I cannot reach the screen where I would normally accept the invitation, so I am unable to join the team.

Steps to reproduce:
1. Receive a team invitation email from HackMD.
2. Click the "Accept invitation" link in that email.
3. The browser opens the invitation URL on hackmd.io.
4. A "Forbidden" page is shown instead of the invitation acceptance screen.

> Screenshot:

#### Expected behaviour
After clicking the invitation link, I expect to land on the invitation acceptance page where I can review the team and join it, and then be redirected into the team workspace as a new member.

#### Environment
I use HackMD on:

**Desktop**
- OS: macOS
- Browser: Google Chrome
- Browser 148.0.7778.96

#### Additional context
- I am logged in to HackMD with the same email address that received the invitation: yes
- The invitation email was received on: 2026-05-13
- I have tried:
- logout / login
- with safari
- ~~clear cookies~~
- Incognito window
— same "Forbidden" page appears.